1. Field of the Invention
The present invention relates to a cement product containing magnesium oxide and a phosphate, and to a mortar formed from the cement product and adapted to be used for various commercial and industrial applications, especially to patch holes in weight-bearing surfaces such as highways, driveways, bridgedecks, and the like.
2. Description of the Prior Art
A cement is a particulate composition that sets up and hardens to a strong, dense monolithic solid upon being mixed with a liquid and allowed to stand. Cements find utility in many areas of commerce, especially in the construction industry, where they are often used in conjunction with various aggregate materials to form such articles as concrete slabs, etc. Cements are also used for highway construction, and in highway patching operations. For highway patching, quick-setting cements, i.e., those which develop an adequate set strength rapidly, are required so that road closing times can be kept to a minimum.
Organic cement compositions, containing polymers which cross-link or cure rapidly when mixed with an appropriate liquid component, are employed for various purposes, and polymer concretes based on furfuryl alcohol, polyester-styrene, and methyl methacrylate have been developed for roadway and runway repair. However, except in cases in which the end-use requirements are highly specialized, organic cements are at an economic disadvantage when compared to inorganic cements. The technology of quick-setting inorganic cements, has focussed largely on the phosphate system, in which a basic component, usually magnesium oxide, reacts with phosphoric acid or a salt thereof in the presence of water. One of the problems encountered with this system, however, is that the reaction is often so fast that the mortar does not remain workable long enough to enable a user to comfortably work it into place, e.g., by trowelling. Thus, while the cement must form a rapid-setting mortar, the rapid setting must not begin until a long enough lead time, at least about ten minutes in most circumstances, has elapsed.
Another important requirement of cements to be used for patching roadways is a compressive strength adequate to support vehicular traffic soon after the mortar has hardened in place, e.g., in about an hour. Until now, the prior art has not had available phosphate cements which can be effectively and systematically controlled with respect to the working time of mortars made therefrom, more particularly without causing unacceptable deleterious effects on the mortar's rate and degree of strength development after placement, an important consideration in road patching.
In the patching method described in U.S. Pat. No. 3,821,006, a patching mortar is prepared by adding water to 7-50 parts of a mixture of minus 100 mesh dead-burned magnesium oxide particles with an acid phosphate salt, and 93-50 parts of an inert aggregate. The acid phosphate salt is monoammonium, monosodium, or monomagnesium phosphate. The ammonium phosphate mortar has a short working time, i.e., less tha six minutes, which is said to be extendible by the addition of sodium chloride or by cooling the dry mix and water before mixing. However, the addition of sodium chloride carries the penalty of reduced compressive strength, which is more pronounced with larger additions. Cooling the dry mix and water is impractical. Increasing the particle size of the magnesia to minus 80 mesh lengthened the working time but decreased the compressive strength to a level deemed undesirable for highway patching. Cements containing monomagnesium and monosodium phosphates produced mortars having short working times and extremely low compressive strengths after curing.
The method of making a fast-setting concrete structure described in U.S. Pat. Nos. 3,879,209 and 4,059,455 also is based on the reaction between magnesium oxide and ammonium phosphate. However, in this instance, the ammonium phosphate to be mixed with magnesium oxide is in an aqueous solution containing polyphosphates. While the specific setting time and early strength depend on the polyphosphate content, these compositions are characterized by short working times, generally 4-5 minutes. Moreover, the highest early strengths are shown to be tied to the shortest working times, so that this system would appear to be unsuited to the achievement of longer working times, e.g., about 10 minutes, in combination with satisfactory early compressive strengths.
U.S. Pat. No. 4,152,167 also describes a composition containing magnesium oxide, ammonium phosphate, and a polyphosphate. In this case, however, the magnesium oxide and the phosphates are in a dry matrix-forming composition, which can be admixed with aggregate or filler material to provide a product which, when mixed with water, sets to form a solidified mass. The dry mix consists of 30-66% dead-burned magnesium oxide, 25-59% monoammonium phosphate, and 1-27% of a water-soluble polyphosphate. Although the composition is said to be readily workable prior to setting, and is described as not generating an undesirable exotherm and as setting to form a solidified mass of high compressive strength, the exemplified mixtures remained fluid for only 1 to 8 minutes, and the length of time required for the hardened mortar to achieve sufficient strength to support vehicular traffic when used in a road patch is not disclosed. The only composition which is shown to have a compressive strength of more than 13,800 kPa after curing for one hour was fluid for only one minute and hardened thoroughly in about 4 minutes.
The magnesium oxide-ammonium phosphate compositions described in U.S. Pat. No. 3,960,580 contain an oxyacid of boron, or a salt or ester of such an acid, as a retardant of the setting time. The one-component product, like the product described in U.S. Pat. No. 4,152,167, requires only the addition of water for activation. The two-component product, like that described in U.S. Pat. Nos. 3,879,209 and 4,059,455, contains an ammonium phosphate solution which includes polyphosphates. Although the boron additive is said to have extended the setting time of the compositions, there is no indication of whether the setting times shown represent working time, i.e., whether the compositions were fluid enough to be mixed and worked into place over the entire disclosed period of time. Moreover, the boron additive dramatically lowered the early strength of the compositions so that road closing times of several hours or days could be necessary if the compositions were employed to patch roadways.
Phosphate cements based on aluminum phosphate as the phosphate component heretofore have been described, for example, in such fields as pigment-containing adhesives and rock bolt grouts. For example, U.S. Pat. No. 2,450,952 describes a dry cement mixture of components reactible upon admixture with water to form an adhesive, the components consisting of a water-soluble aluminum phosphate and magnesia, olivine and/or magnesium silicate. The magnesium compound constitutes about 65-90% of the dry mixture, while the weight ratio of the magnesium compound to the aluminum phosphate is 2:1 to 8:1. The magnesium oxide is used in combination with olivine (a silicate), and the rapid set is said to be slowed down by transforming the magnesium oxide grain or its surface into magnesium silicate, or coating the grain with silicic acid.
The phosphate rock-bolt grouting system described in U.S. Pat. No. 4,174,227 includes products containing aluminum phosphate, magnesium oxide, aggregate, and water which harden around a rock bolt usually in about 1-2 minutes and reach a pull strength of at least about 175 kilograms per centimeter of bolt-anchoring length usually in 5-10 minutes. Very little working time, e.g., about 15-45 seconds, is required, and the fastest strength development possible is needed to prevent mine roof failure. These compositions are not adapted to be road-patching compositions because of their extremely short working time. Moreover, their rapidly developed high pull strength is not required of road-patching compositions. The ability of aluminum phosphate to form a crosslinked polymeric network in the hardening reaction has been suggested as being related to the rapid development of high pull strength in rock bolt grouts containing aluminum phosphate, thereby causing this system to be associated more particularly with applications requiring fast strength development.
The magnesium oxide used in the rock bolt grouts of U.S. Pat. No. 4,174,227 has a surface area of preferably at least one, and in the range of up to about 40, square meters per gram, i.e., it is the so-called "chemical grade" magnesium oxide. It constitutes about from 5 to 35, and preferably about from 8 to 25, percent of the total weight of the grout. The aggregate constitutes about from 30 to 70 percent of the total weight of the grout, and the aggregate particles have a minimum dimension no larger than about 3 millimeters. The grouting compositions of U.S. Pat. No. 4,174,227 can contain magnesium oxide of low surface area only if the magnesium oxide particle size is small, i.e., more than about 95 percent of the particles pass through a 200-mesh screen, and its concentration is high, i.e., 18 to 25 percent.